Add README and proper favicon/PWA icon set

README covers what alembic does, prerequisites, a full deploy quickstart,
credential setup, and a page-by-page usage tour, written for someone
running Docker but not necessarily comfortable coding or sysadmin work.

Favicon/icons were prepared in a separate theme scratch dir but never
wired in; moved them into app/static and pointed base.html at the full
icon set (SVG, PNG fallbacks, apple-touch-icon, manifest) instead of the
single inline data-URI SVG it had before.

app/templates/base.html

@@ -4,7 +4,12 @@
   <meta charset="utf-8">
   <meta name="viewport" content="width=device-width, initial-scale=1">
   <title>{% block title %}alembic{% endblock %}</title>
-  <link rel="icon" href="data:image/svg+xml,%3Csvg xmlns='http://www.w3.org/2000/svg' viewBox='0 0 24 24'%3E%3Cdefs%3E%3ClinearGradient id='g' x1='0' y1='0' x2='24' y2='24'%3E%3Cstop offset='0%25' stop-color='%237dd3fc'/%3E%3Cstop offset='50%25' stop-color='%23a78bfa'/%3E%3Cstop offset='100%25' stop-color='%23f9a8d4'/%3E%3C/linearGradient%3E%3C/defs%3E%3Cpath fill='url(%23g)' d='M9 2.5h6v5.2l4.3 8.6a2.6 2.6 0 0 1-2.33 3.76H7.03A2.6 2.6 0 0 1 4.7 16.3L9 7.7V2.5z'/%3E%3C/svg%3E">
+  <link rel="icon" type="image/svg+xml" href="/static/icon.svg">
+  <link rel="icon" type="image/png" sizes="16x16" href="/static/favicon-16.png">
+  <link rel="icon" type="image/png" sizes="32x32" href="/static/favicon-32.png">
+  <link rel="icon" type="image/png" sizes="48x48" href="/static/favicon-48.png">
+  <link rel="apple-touch-icon" href="/static/apple-touch-icon.png">
+  <link rel="manifest" href="/static/site.webmanifest">
   <link rel="preconnect" href="https://fonts.googleapis.com">
   <link rel="preconnect" href="https://fonts.gstatic.com" crossorigin>
   <link href="https://fonts.googleapis.com/css2?family=Space+Grotesk:wght@500;600;700&family=Manrope:wght@400;500;600;700;800&display=swap" rel="stylesheet">